Traditionally, soft, hollow toy parts such as doll heads were made by either by slush molding, dip molding, or rotocasting processes using latex or PVC, or by blow-molding processes using thermoplastic elastomers. These traditional processes, however, are not able to make hollow parts with controllable and precise wall thicknesses and opening dimensions. Nonetheless, due to the softness and easy processing of PVC, slush molding and rotocasting processes using PVC have been the primary process and the primary material for making one-piece hollow doll heads with direct hair rooting in the past half century.
In the last decade, questions regarding the environmental safety of PVC have lead some toy manufacturers to elect to discontinue PVC use in toys and dolls. Use of materials other than PVC has created the need for new processes for making soft hollow toy parts with controllable and precise wall thickness and opening dimensions.
In U.S. Pat. No. 6,403,003 (“'003 patent”), the complete disclosure of which is incorporated herein by reference, a method is provided for making a soft hollow part for a toy, such as a head for a doll. The method includes providing an injection moldable flexible thermoplastic elastomer, and providing a mold for the head part. The mold includes exterior mold parts and an interior core part, wherein the exterior mold parts are spaced apart from the interior core to define a cavity in the shape of the hollow part to be formed. The thermoplastic elastomer is injected into the mold cavity to form the hollow head. The head has an opening for removing the interior core, but the dimension of the core is larger than the dimension of the opening through which the core must be removed. After the head is injection molded, the exterior mold parts are opened to release the head and interior core, and then the interior core is removed from the head by extracting it through the opening in the head.
Preferably, the method disclosed and claimed in the '003 patent uses a non-PVC thermoplastic elastomer. In particular, a family of flexible thermoplastic elastomers known as S-B-S (Styrene-Butadiene-Styrene), S-I-S (Styrene-Isoprene-Styrene) and SEB-S (Styrene-Ethylene/Butylene-Styrene), marketed by Shell Chemical Company under the name KRATON, is preferred because of its resilience, strength and durability, as well as the ability to be painted and passing the general paint adhesion requirements for toys and dolls applications. An S-B-S copolymer, such as the copolymer marketed as KRATON D, is a particularly preferred elastomer. This copolymer is reported to be environmentally friendly and safe.
In a second aspect of the invention of the '003 patent, a mold is provided for making an injection molded hollow article, such as a doll head, using a thermoplastic elastomer. The mold includes an exterior mold portion and an interior core assembly. The exterior mold portion includes at least two separable sections for enclosing the interior core assembly. The interior core assembly fits inside the exterior mold portion. The interior core is spaced apart from the exterior mold portion to define a variable-thickness cavity having the shape of the hollow article to be molded and into which a melted thermoplastic elastomer may be injected to form the hollow article. The hollow article takes the shape of the cavity and includes an opening through which the interior core is removable. The interior core assembly has a cross-sectional dimension that is larger than the dimension of the opening in the hollow article from which it must be removed after the article is molded. Accordingly, this mold is useful for making a variety of hollow toy parts that have a narrow opening, such as heads, feet, or hands.
The '003 patent provides a method of making a doll head without an observable part line. When the head is injection molded, a part line is created at the junction of two exterior mold halves. The part line extends in a continuous line around the top of the head above the ears. Hair-material is rooted to the top of the doll head above and below the part line with a sufficient density such that the part line is not observable to an ordinary observer holding the doll at arms length.
Preferably, the invention of the '003 patent takes advantage of certain mold structures to make the soft hollow parts and dolls heads. A two-part solid core design with a mushroom-shaped ejector pin, double-ejection system may be used to locate the parting line at the top part of head within the hair rooting area, as shown in FIGS. 6–8 of the '003 patent, to make the part line not observable to the ordinary observer.
While the process of the '003 patent enables the manufacture of soft hollow toy parts and doll heads and provides for controllable and precise wall thickness at specific locations, the exact wall thickness formed is sometimes greater than desired. Because the thermoplastic elastomer is not forcibly dispersed against the top and bottom mold halves before it sets to form the doll head, the head forms with a thickness equivalent to the entire volume of the cavity. Due to tool structure limitations, the process of the '003 patent produces, in some cases, doll heads with sections that are thicker than otherwise desired. For example, the doll chin might be too thick. This in turn limits the deformability of the doll head, hence hindering its application in squeeze toys and dolls in which appendage deformation accentuates a realistic appearance. Additionally, formation of undesirably thick sections requires application of a higher holding pressure and a longer holding time to in order to cool and solidify the article without the development of shrinkage marks.
The need exists, therefore, for a thermoplastic molding process useful in the manufacture of hollow, deformable articles such as doll heads which have a relatively uniform and minimum thickness and which are free of manufacturing flaws such as shrinkage marks. Ideally, such a process would facilitate the removal of a manufactured article from a mold without damage to any features of the article.